Shartrughna and Shrutakirti.
Lakshman and Urmila are talked about because of Urmila's sacrifice, and Bharat-Mandavi are also given attention because of the incidents involving Bharat and the raaj-gadi. Bharat took a vow to live the life of an ascetic for years and to live as Ram would in the forest. Mandavi didn't accompany him although she wanted to, and her sacrifice is also talked about, as I have seen in RS's Ramayan.
But Shatrughna and Shrutakirti are rarely given dialogues and portrayals, and very little about their characters is analyzed or taken into account...it's like they're just there.
^^ That's just individually. Nothing really is said about them as a couple.
I'm really interested in learning more about these two...wish I could just call them up and ask for an interview